Does anyone know a decompiler?

A decompiler is the name given to a computer program that performs the reverse operation to that of a compiler. That is, it translates a file containing information at a relatively low level of abstraction (usually designed to be computer readable rather than human readable) in to a form having a higher level of abstraction (usually designed to be human readable).

The term "decompiler" is most commonly applied to a program which translates executable programs (the output from a compiler) into source code in a (relatively) high level language (which when compiled will produce an executable whose behavior is the same as the original executable program). By comparison, a disassembler translates an executable program into assembly language (an assembler could be used to assemble it back into an executable program).

Decompilation is the act of using a decompiler, although the term can also refer to the decompiled output. It can be used for the recovery of lost source code, and is also useful in some cases for computer security, interoperability, error correction, and more (see "Why Decompilation"). The success of decompilation depends on the amount of information present in the code being decompiled and the sophistication of the analysis performed on it. The bytecode formats used by many virtual machines (such as Java's JVM) often include extensive metadata and high-level features that make decompilation quite feasible. Machine code has typically much less metadata, and is therefore much harder to decompile.

Yes, but you didn't mention the language. For Java, you can use DJ Java Decompiler or Decafe. Just google with either of these.